expression: remove the usage of "TypeClass" in "builtin_cast.go" (#4570)
expression: remove the usage of "TypeClass" in "builtin_cast.go" (#4570)

* expression: remove the usage of "TypeClass" in "builtin_cast.go"

8cbx/tidb,ngaut/tidb,ming-relax/tidb,yuanwhy/tidb,ngaut/tidb,c4pt0r/tidb,c4pt0r/tidb,kyle-wang/tidb,wusphinx/tidb,qiuyesuifeng/tidb,zimulala/tidb,cosmtrek/tidb,louishust/tidb,mccxj/tidb,Cofyc/tidb,CodeRushing/tidb,XuHuaiyu/tidb,CodeRushing/tidb,XuHuaiyu/tidb,pingcap/tidb,spongedu/tidb,mccxj/tidb,kyle-wang/tidb,pingcap/tidb,Nify/tidb,ngaut/tidb,ngaut/tidb,pingcap/tidb,kyle-wang/tidb,kyle-wang/tidb,pingcap/tidb,c4pt0r/tidb,ming-relax/tidb,coocood/tidb,c4pt0r/tidb,8cbx/tidb,tiancaiamao/tidb,wusphinx/tidb,c4pt0r/tidb,ngaut/tidb,coocood/tidb,louishust/tidb,Cofyc/tidb,cosmtrek/tidb,kyle-wang/tidb,kyle-wang/tidb,pingcap/tidb,ngaut/tidb,zimulala/tidb,yuanwhy/tidb,EvilMcJerkface/tidb,EvilMcJerkface/tidb,yangwenmai/tidb,qiuyesuifeng/tidb,Nify/tidb,spongedu/tidb,c4pt0r/tidb,yangwenmai/tidb,tiancaiamao/tidb,pingcap/tidb